Hitherto, the use of radar principles for such a purpose has been deemed impractical, owing to the difficulty of measuring range distance with high enough precision. It is known to transmit short radio pulses and to sense the echo reflection of those pulses in the short period between pulse transmission as a means of measuring distance. A problem is noise from spurious reflections and the assumption is that the carrier medium transmits the signal either way with a definite and known propagation velocity. Usually this is the speed of light in air, for which a one-way distance of 0.3 meters corresponds to a transit time of 1 nanosecond in vacuo and a somewhat longer duration in solids and liquid substances.
The precision of such a measurement is a function of the precision with which this short time period can itself be measured. Although the pulse repetition rate can be high and an average of numerous such measurements obtained, the resolution or sensitivity of the measurement is limited to the response time of that basic time measure.
It is known that a continuous electrical signal of high frequency can be propagated in a modulated form and the distance assessed from the phase shift as between the modulation of the transmitted signal and the received signal. If used over a fixed and known distance, such techniques can measure the speed of light. The accuracy of the measurement is then a function of the degree of precision governing the phase shift measurement.
By way of example a modulating frequency of 100 MHz corresponds on a round-trip basis to a range of 1.5 meters per wavelength of phase shift. Thus, if the transmitted signal is propagated continuously and the phase shift is measured continuously, knowledge of the precise phase shift can give a range measure somewhere in a scale unit of 1.5 meters, but the problem then is that it is not known how many scale units exist between the transmitter and the target reflector.
To measure absolute distance, as opposed to integrating a measure of time rate of change of distance and so deduce a change of distance, the wavelength of the modulating signal has to be greater than the range of uncertainty of the position of the target. The smaller this wavelength, the more useful the system is for measuring short distances with high precision, as for control measurements confined to the ranges between relatively moving parts of apparatus in an industrial application. However, then the time or phase measurement is more demanding.
Primarily, this invention addresses the latter problem in aiming to provide a cheap and reliable technique of radar type scanning at very short ranges measured in meters and centimeters to accuracies that are quite small fractions of a millimeter.
Typical applications are for data acquisition, for example, by the remote sensing of the precise liquid level in a storage tank subject to hazardous conditions or by the remote sensing of the position of a scanning probe as it is manipulated over the surface of a body or structure being surveyed. Indeed, the invention has particular application for short-range distance measurement as well as precise long-range survey work. Hitherto, radar-type scanning methods have lacked the precision needed for such applications.
To achieve at moderate expense the very high precision and reliability demanded using speed-of-light ranging methods, the inventors have addressed a problem confronting those who have sought to ease the phase-shift measurement by frequency down conversion. The problem is one of avoiding the very low frequency oscillations which are easily set up where two oscillators cooperate at nearly equal frequencies. This affects the stability of any distance indication.
Long distance ranging methods, such as communication with a space craft, have been suggested in which the signal transmitted is received at a remote receiver in a space craft having its own oscillator and retransmitted back to source at a different frequency.
Short distance ranging methods, such as liquid level sensing in a container, have been suggested in which the transmitted signal is reflected and sensed by a receiver closely adjacent the transmitter. In one such method two oscillators operating at slightly different frequencies were proposed, one serving to modulate the transmitted signal and the other operating in conjunction with the detecting receiver to demodulate the received signal.
Important, however, from the viewpoint of reliability and precision, is the need for the relative phase information carried by the transmitted and received signals to be preserved by processing both signals at the point of comparison at the same frequency in an identical way. This is best achieved if all the electronic data processing occurs in the same circuit unit. Furthermore, as will be seen from what is proposed in the subject invention, it is preferred that the generation of two slightly differing frequencies, one for the transmitter-receiver circuit and one for the frequency down-conversion analysis should be phase-locked to avoid the spurious low frequency effects which otherwise make the distance measure unreliable.
The inventors are aware of a technique for measuring the relative speed of light as between parallel transmission along two different propagation channels, by sensitive phase difference measurement and use of a common signal source having a frequency monitored under the control of a phase-locked loop responsive to the reduced frequency of a frequency-shifted version of the primary signal. The object of this prior art technique, as described by D. R. Gagnon, D. G. Torr, P. T. Kolen and T. Chang in an article entitled `Guided-wave measurement of the one-way speed of light` in Physical Review A, 38, 1767 (1988), is essentially to keep the test frequency constant in spite of movement and reorientation of the apparatus. The invention to be described involves entirely different principles, in that the phase-locking feature of the implementations involving the subject invention has the object of enhancing the precision of the phase measurement representing distance rather than merely controlling the stability of the frequency of the primary signal. Even though the latter is essential to both this invention and the prior art disclosure and though crystal oscillators are used as stable control sources, the phase measurement technique, which is vital to the measurement of the lightspeed and is the subject of this invention, is quite different from that disclosed in the prior art.
Reference may also be made to Wikland and Ericson U.S. Pat. No. 4,229,102 which applies the principle by which two oscillators operating at slightly different frequencies are used, one as the primary oscillator determining the frequency of the transmitted and the reflected signal, and the other as a secondary oscillator, the signal of which is separately mixed with the transmitted and received signals to develop a measure of phase on an extended time scale at the frequency difference of the two oscillators. This disclosure refers to phase-locking but in the context that two signals are compared which may each be subject to drift affecting phase. The object is to make a separate calibrating measure and so compensate for the phase error involved when the signals are used to perform a distance measure over a calibration range of an internal optical path. Thus, the phase-locking in this prior art disclosure is effected through a double measuring operation and involves circuit components positioned away from the oscillators to sense a phase error which takes into account other circuit phase shifts, whereas the invention to be presented below concerns an absolute phase-lock of one oscillator by the action of digitally synthesizing from each of the oscillators a signal at the difference frequency and regulating one oscillator to keep these synthesized signals in precise time accord.
The disclosure in Hullein and Fribault U.S. Pat. No. 4,639,129 also uses two oscillators and mixer techniques to transfer the phase measure to a lower frequency, but is concerned with techniques for sampling the phase measurement as an indication of the test distance and averaging the measurement as a function of its variation.
The assumption in such disclosures is that two oscillators are stabilized suffiently to operate with no significant drift during the time lapse corresponding to the lower frequency phase measurement of the mixer output. Any such drift can be taken into account by comparing measurements of such low frequency phase as between the two non-transmitted signals and the comparison of the received and non-transmitted signals. However, in such prior art proposals the emphasis is placed on the interpretation of phase measurement by signal calibration and correction at the output stage, whereas in the invention to be described a very positive phase-lock as between the two oscillators is deemed to be of paramount importance and other spurious phase shifts are compensated by the identical matching of the circuit components in the two signal channels.
In this way, the invention to be described not only achieves a resolution and measurement precision that is superior but does this by an inexpensive combination of circuit components, which extends the range of practical application of such optical radar measurement methods.